AB165 Bassman

[TR]

I just picked up a 68 bassman that appears to be the same design as this guy's:

I haven't been able to find an original layout diagram or schematic that shows pins 4 and 9 on the preamp tubes all being interconnected with each other and with pins 2 and 7 on the power tubes, and end at the pilot light. Can anyone tell me what these connections are doing? Is it an original design or a mod? Thanks.

[hasserl]

That is the filament circuit, it is original. Each tube has a filament that heats the cathode. If you look closer you'll see pin's 4 & 5 of each preamp tube are joined together. This is a parallel circuit, they can also be wired in series. Each leg of the circuit will have around 3.1 volts wrt ground, or around 6.3 volts between them.

[52 Bill]

The wires that connect pins 2-7 on the power tubes and pins 4/5-9 on the preamp tubes are the heater supply connections. They aren't shown on the schematics and layouts for simplification. This is definitely not a mod.

[TR]

Thanks for the info.

[TR]

Thanks, I appreciate it.